Definition
Parallax is the difference in apparent direction of an object as seen from two different vantage points. Diurnal parallax arises from Earth's rotation; annual (stellar) parallax from Earth's orbital motion.
Distance ladder
Stellar parallax π (in arcseconds) gives distance d in parsecs: d = 1/π. Gaia measures parallaxes to microarcsecond precision for over a billion stars.
